Auto-Locking Overhauser Field to the Sweet Point for an Electron Spin by Quantum Weak Measurement of Nuclear Spins*

The sweet points such as the clock transitions (CTs) are important for quantum information processing (QIP) and quantum sensing technologies. Being inherently robust to the fluctuations of the environment, they provide almost the best options to counteract the decoherence of the electron spin. The random fields from the environment (Overhauser fields) that lead to the decoherence of the electron spin, however, can also destroy the sweet points, which sets up a big obstacle to the applications. Here we propose a new dynamical nuclear polarization (DNP) scheme that can auto-lock the Overhauser field to the sweet point of the nitrogen vacancy (NV) center. The DNP channel is established by combining the bath state dependent microwave (MW) control, the conditional flipping of the nuclear spins and the optical pumping of the NV center electronic spin. The mechanism can also be understood as sequential weak measurements of the nuclear spins followed by entropy dumping through initialization of the electron spin.